High temperature abrasive resistant heat exchanger

ABSTRACT

A high temperature tube and shell vertically positioned heat exchanger including a plurality of tube sheets dividing the interior of the shell into three consecutive chambers. Ceramic tubes are vertically hung from hemispherical seats in an upper tube sheet and extend downwardly through loose fitting porous inserts which line perforations in the lower tube sheets. A hot fluid flows from the bottom chamber upwardly through the ceramic tubes, a cool fluid flows through the upper chamber across the tubes, and a third fluid is injected into the intermediate chamber at a pressure higher than that of the fluid mediums in the upper and lower chambers. The third, high pressure fluid flows through the porous inserts and into the other chambers forming a dynamic seal which also allows unrestricted axial motion of the tubes and limited laterial motion of the bottom of the tubes.

This is a division of application Ser. No.49,688, filed June 18, 1979,subsequently issued as U.S. Pat. No. 4,279,293.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to shell and tube-type heat exchangers, and moreparticularly to a construction utilizing ceramic components transportinghigh temperature fluid mediums.

2. Description of the Prior Art

In combined gas turbine-coal gasification generating plant designs, rawfuel gas, prior to being burned in the gas turbines, has to besufficiently cleaned in order to remove particulate matter and chemicalimpurities such as H₂ S, COS, HCN, CS₂, HCl, KCl, KOH and FE(OH)₂ tovery low levels, as an impure fuel gas detrimentally affects turbinelife. Several options for cleaning fuel gas for gas turbine applicationsare available, or are presently being considered.

For example, the raw fuel gas can be cooled by direct water sprays to atemperature below the boiling point of water, at the operating pressure,and particulate matter and chemical impurities removed by commerciallyavailable so-called "wet" processes. Also, raw fuel gas can be cooled ina waste heat boiler, with or without a water quench, then cooled furtherby direct water sprays, and particulate matter and chemical impuritiessubsequently removed by commercially available wet processes. Althoughthese processes are presently being utilized commercially, they resultin gross generating system inefficiencies because cooling of gas inthese systems involves large temperature differences. Also beinginvestigated are processes which remove the particulate matter andchemical impurities directly from the high temperature fuel gas.However, viable commercial technology for high temperature gas cleaningis not presently available.

Another option is to cool the raw fuel gas by heat exchange with a cleanfuel gas, followed by further cooling such as by direct water sprays,followed by removal of the particulate matter and chemical impurities bycommercially available processes. This latter process appears to have ahigh potential for widespread use in coal gasification-gas turbine powergenerating plants. However, associated with use of such high temperatureheat exchangers are a number of concerns including the corrosive effectof the chemical impurities in the raw fuel gas on commercially availablealloys. Additionally, a high temperature heat exchanger is highlysusceptible to erosion of the heat transfer surfaces by particulatematter in the raw fuel gas stream. And, the heat transfer surfaces arefurther subject to fouling by coal tar deposition and cracking.

The corrosion concerns can be alleviated to some extent by use of exoticmetals and metal alloys, primarily for the tubes. Metals and alloyswhich can withstand the chemical attack are not immune to the erosion bysolid particulates in the gas. Ceramic materials, on the other hand, areeffectively resistant to both corrosion by chemical impurities and alsoto erosion by particulate matter, and thus appear to be the most viablealternative. However, practical application of ceramic materials in ahigh temperature heat exchanger is complicated by the relatively lowstrength and low ductility of ceramic tubes, as well as the difficultyencountered in fabricating long ceramic tubes which are sufficientlystraight. The application is further hampered by the differentialthermal expansions between, for example, ceramic tubes and metals usedin the construction of a heat exchanger pressure shell. Adequatesolutions to these concerns have not appeared.

It is thus desirable to provide a high temperature heat exchanger,particularly for coal gasification-gas turbine applications, whichovercomes the discussed concerns. It is further desirable to provide aheat exchanger which effectively utilizes ceramic components, overcomingthe strength, ductility, lack of straightness and differential thermalexpansion characteristics heretofore detrimentally associated withceramic components.

SUMMARY OF THE INVENTION

This invention provides a heat exchanger, particularly useful forcombined coal gasification-gas turbine application, which allows hightemperature heat exchange among fluid mediums, at least one of which ishostile as a result of its chemical nature and containment of chemicalimpurities and particulate matter. The invention further allows use ofceramic components, effectively alleviating previous limitationsresulting from low ductility, low strength, lack of straightness andthermal expansion characteristics of ceramic components for heatexchanger utilization.

In a preferred form the heat exchanger is a vertically oriented tube andshell type. The shell is metallic and the tubes are ceramic, laterallysupported along their length by a plurality of tube sheet structureswhich divide the shell interior into three main chambers. Portions ofthe tube sheets, such as cylindrical inserts fitting about the ceramictubes, are porous and flexible. The flexibility of the inserts allowsaxial expansion and contraction of the tubes without generatingexcessive stresses, and the porosity of the inserts provides controlledfluid communication between selected chambers.

A contaminated hotter fluid medium, such as raw fuel gas, enters thebottom chamber and passes through the interior of the tubes. A secondcooler fluid medium, such as clean fuel gas, passes into and through anupper chamber, across the tubes, absorbing heat from the raw fuel gas. Athird intermediate chamber is interposed between the upper and lowerchambers, and a clean gaseous medium, which may be the same as themedium in the upper chamber, is injected into the intermediate chamberat a higher pressure than the pressure in either of the other twochambers. This intermediate medium, because of its higher pressure,passes into the other two chambers through the porous inserts, thusforming a dynamic fluid seal between the lower and upper chambers.

The ceramic tubes additionally have integral generally spherical flangesat their upper ends which seat in matingly configured hemisphericalperforations in an upper tube sheet so as to form a ball joint typeconnection. The tubes are thus able to accommodate axial expansion andsignificant curvature of the ceramic tube without generating excessivestresses, particularly at the flange. The preferred tube material isdense silicon carbide (SiC), and the porous inserts are preferablyfabricated from a dense mat of high alloy wire, textured similar todense steel wool.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature and additional features of the invention willbecome more apparent from the following description, taken in connectionwith the accompanying drawings, in which:

FIG. 1 is a vertical section through a heat exchanger in accordance withthe invention;

FIG. 2 is a section view of the joint between a tube and tube sheet inaccordance with the invention; and

FIG. 3 is a schematic of a portion of a system utilizing a heatexchanger in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown a high temperature tube and shelltype heat exchanger 10. The shell assembly 12 includes a body 14, upperhead 16 and lower head 18. The heads 16, 18 are joined to the body 14through flanges 20, to form a pressure bearing assembly. An upper 22 andlower 24 perforated tube sheet are preferably supported between theflanges 20, although the tube sheets 22, 24 can alternatively besupported from any of the three components making up the shell assembly12. An intermediate tube sheet 26 is supported within the shell,preferably closer to the lower tube sheet 24 than to the upper tubesheet 22.

A plurality of ceramic tubes 28 (one shown), preferably of dense siliconcarbide (SiC), are supported vertically within the shell assembly 12.Tubes comprised of other ceramics such as beryllium oxide, densealumina, zirconia and its oxides, as well as cermets, mixtures ofsintered oxides and metals, can also be utilized. The upper end of eachtube includes a generally spherical flange 30, which is integrallyformed with the tube 28 and which seats in a matingly configuredgenerally hemispherical perforation 32 in the upper tube sheet 22, asshown in additional detail in FIG. 2. Hemispherical refers to thegeneral shape of, for example, the perforation, although the bottomportion is flattened where the body of the tube 28 passes through. Thisarrangement provides a ball joint type seat which, coupled with theflexible lateral support of the lower portion of the tube, discussedfurther below, allows a degree of non-linearity of the ceramic tube 28so as to alleviate detrimental stresses. A plate 34, preferably affixedto the tube sheet 22 by fasteners 23, is positioned atop the tubes 28 toprovide a force which holds the flanges 30 in the perforations 32. Thetubes 28 extend downwardly through the intermediate 26 and lower 24perforated tube sheets.

The tube sheets divide the interior of the shell assembly 12 into aplurality of chambers. A hot first fluid medium, such as raw fuel gasincluding particulate matter, enters a first chamber 36 through an inletnozzle 38, flows upwardly through the interior of the tubes 28, and isdischarged from an outlet chamber 39 through outlet nozzle 40. A fluidmedium to be heated, such as a clean fuel gas, enters a second chamber42 through an inlet nozzle 44, flows about the tubes, preferably in aserpentine pattern controlled by a plurality of flow baffles 46,absorbing heat energy from the raw gas within the tubes, and isdischarged through an outlet nozzle 51. In a typical application, rawfuel gas from, for example, a coal gasification reactor, enters thetubes at approximately 1750° F. and at a pressure of fifteen to fortyatmospheres, and is discharged at approximately 650° F.; the clean fuelgas enters the shell assembly 12 at approximately 230° F. and isdischarged, for example to a gas turbine, at 1430° F.

Interposed between the chambers 36 and 42 is an intermediate chamber 48bounded by the tube sheets 24, 26. A clean intermediate fluid medium isinjected, through inlet nozzle 50, into the intermediate chamber 48 at apressure which is higher than that in either chamber 36 or chamber 42,preferably at a differential of between two and ten psi. Theintermediate medium can, for example, be clean fuel gas at a pressurehigher than the fuel gas flowing through chamber 42, for example,comprising a clean cool fuel gas at 230° F. passed through a compressor52 and then inlet nozzle 50, as shown in FIG. 3.

The primary structure of the tube sheets 24 and 26 is preferablyimpermeable to the contiguous fluid mediums, and selected portions aboutthe ceramic tubes 28 are permeable to the high pressure medium injectedinto the chamber 48. The permeable portions preferably take the form ofgenerally cylindrical inserts 54 disposed through perforations 56 in thetube sheets 24, 26. The inserts 54, which can comprise cylindricalcomponents fabricated from wire made from such alloys as, for example,Stellite 6B or Haynes 188 (commercially available from the CabotCorporation), or Thermalloy 63WC (commercially available from the AbexCorporation) are sized to flexibly receive the ceramic tubes 28 whileallowing a degree of axial motion or eccentric position with respect tothe tube sheet perforations 56 without developing excessive stresses inthe tubes 28. The inserts thus not only provide for substantiallyunrestrained axial expansion and contraction of the ceramic tubes 28relative to the shell assembly 12 and affixed components, but also forma dynamic seal preventing direct communication between the chambersabove 42 and below 36, the intermediate chamber 48. The lower tubesheets also provide a radial restraint for the bottom portion of thetubes 28. For fabrication, the tubes 28 can be inserted through theperforations 56, and the porous flexible insert material subsequentlypacked about the tube. The inserts can also be positioned prior to tubeinsertion. Where high temperature fluids are utilized the outer portionsof the insert material may self-weld to the metallic tube sheetperforation during operation. However, no similar reaction will occurbetween the metallic insert and the ceramic tube, maintaining theflexible relation.

In order to maintain the preferably carbon steel shell assembly 12 atacceptable operational temperatures, an insulating layer 58 is provided,for example, a monolytic refractory such as commercially availablemixtures of Al₂ O₃, MgO and SiO₂. To prevent contamination of the cleanfuel gas by spalling of the refractory layer 58, the layer 58 ispreferably lined with a jacket 60 of high temperature alloy, for example1N-657 (commercially available from the Huntington Alloys Corporation)or 310 stainless steel, which extends between the tube sheet 22 and tubesheet 26. Other interior portions of the shell assembly 12 can also belined, such as the intermediate chamber between the tube sheet 26 andthe tube sheet 24. The tube sheets, being exposed to high temperaturemediums, should also be comprised of similar high temperature alloys.

It will be apparent that the disclosed arrangements can beneficially beapplied to a variety of fluid mediums. For example, a dynamic fluid sealin combination with ceramic or metallic tubes can be utilized inprocesses involving heat exchange among corrosive liquids, such asacids.

Numerous other applications and modifications may be made with theabove-described apparatus without departing from the spirit and scopethereof. It therefore is intended that all matter contained in theforegoing description shall be interpreted as illustrative and not in alimiting sense.

I claim:
 1. A shell and tube heat exchanger comprising:a. a shell; b. aplurality of tubes supported within said shell; c. tube sheets fordefining a first, second and third chamber within said shell and forsupporting said tubes, portions of said tube sheets being porous, saidthird chamber being disposed between said first and second chambers, oneof said tube sheets comprising hemispherical indentations; d. means forinletting a first fluid medium into said first chamber, said firstchamber being in flow communication with the inside of said tubes; e.means for flowing a second fluid medium through said second chamber inheat exchange relation with said first fluid; f. means for inletting athird fluid medium into said third chamber at a pressure higher thanthat of said first and second chambers such that said third fluid passesfrom said third chamber into said first and second chambers through saidporous portions; said tubes comprising an integral spherical flangematingly configured to said hemispherical indentations in said one tubesheet.
 2. The heat exchanger of claim 1 wherein said tubes comprisedense silicon carbide.
 3. A vertically oriented shell and tube heatexchanger comprising:a. an elongated vertically disposed shell assembly;b. an upper tube sheet having a plurality of generally hemisphericallyshaped perforations; c. two lower perforated tube sheets dividing theinterior of said shell assembly into a plurality of chambers including alower inlet chamber, an intermediate chamber and an upper heat exchangechamber, said tube sheets including porous inserts disposed through saidperforations; d. a plurality of ceramic tubes, the upper portion of eachsaid tube comprising a generally spherical flange matingly sized to seatin a corresponding one of said hemispherical perforations; said tubesextending downwardly through, and sized to be loosely received by, saidporous inserts; e. means for restraining upward movement of said upperportion of each said tube; f. means for flowing a hot fluid medium intosaid lower chamber and through said tubes; g. means for flowing a coolfluid medium through said upper chamber in thermal interexchange withsaid hot fluid medium; and h. means for injecting a third fluid mediuminto said intermediate chamber at a pressure higher than the pressurewithin said upper and lower chambers; whereby said third fluid flowsinto said upper and lower chambers forming a dynamic fluid seal whichsubstantially prevents direct communication between said first andsecond fluid mediums.